Ankylosaurus

is a genus of large, heavily armored ankylosaurid dinosaur that lived in western North America during the latest part of the Late Cretaceous Period, approximately 68–66 million years ago. It was among the last non-avian dinosaurs to exist before the Cretaceous–Paleogene extinction event. The genus is known for its extremely robust body, extensive covering of bony armor, broad skull, powerful jaws, and especially its large bony tail club.

The only recognized species is Ankylosaurus magniventris, named by the American paleontologist Barnum Brown in 1908. The generic name Ankylosaurus is derived from Ancient Greek words referring to something "fused" or "bent", alluding to the extensive fusion of bones found in the animal. The specific name magniventris means approximately "great belly", referring to its unusually broad body.

Despite being one of the most recognizable dinosaurs, Ankylosaurus is surprisingly incompletely known. No complete skeleton has yet been discovered, and much of its reconstruction is based on several partial specimens and comparisons with better-preserved relatives.

The first fossils attributed to Ankylosaurus were discovered during expeditions led by Barnum Brown, one of the most prominent American paleontologists of the early twentieth century.

Brown originally discovered material from the Lance Formation of Wyoming in 1906, and additional specimens were collected during subsequent expeditions. In 1908, he formally established the genus Ankylosaurus and described its type species, Ankylosaurus magniventris.

The early reconstruction of the dinosaur was necessarily speculative because the known material represented only parts of the animal. This resulted in some unusual reconstructions that persisted in scientific and popular illustrations for decades.

The limited fossil record also means that some details of the animal's exact proportions remain uncertain.

Discovery and naming

Skeletal Reconstruction

Jiangxi Museum

Fossil record

Fossils attributed to Ankylosaurus have been recovered from several Late Cretaceous formations in western North America.

Important fossil-bearing formations include:

Hell Creek Formation
Lance Formation
Scollard Formation
Frenchman Formation
Ferris Formation

These formations preserve ecosystems dating to approximately the final few million years of the Cretaceous.

The Smithsonian collections, for example, contain Ankylosaurus material from the Lance Formation of Wyoming dating to the Maastrichtian stage of the Late Cretaceous.

Unlike some other ankylosaurs, Ankylosaurus is represented by relatively few specimens. Consequently, many aspects of its anatomy are reconstructed using comparisons with other ankylosaurids.

Description

Ankylosaurus was a large, heavily built quadrupedal dinosaur. Estimates generally place its length at approximately 6–8 metres (20–26 feet), with mass estimates ranging from roughly 4.8 to 8 metric tonnes. Some museum estimates use approximately 8 metres and 8,000 kilograms.

It had a very broad body and relatively short limbs. Its overall shape was low to the ground, giving it a somewhat tank-like appearance.

The most distinctive features of Ankylosaurus included:

a broad, low skull;
a tooth-bearing beak;
numerous bony osteoderms;
protective neck armor;
backward-pointing skull horns;
a massive, reinforced tail;
and a large bony tail club.

The combination of armor and a tail weapon made Ankylosaurus one of the most heavily defended dinosaurs known.

Museum of the Rockies

Ankylosaurus head,

Ankylosaurus magniventris

Skull

The skull of Ankylosaurus was broad and relatively low compared with its length. The posterior region of the skull possessed large horn-like projections. Two prominent horns projected backward from the rear corners of the skull, while additional structures below them projected backward and downward. The skull bones were strongly reinforced and partially fused, contributing to the animal's overall cranial strength. One unusual characteristic of Ankylosaurus concerns its nostrils. Unlike many other ankylosaurs, its external nasal openings were positioned more laterally rather than being directed predominantly forward. The snout was wide and adapted for cropping vegetation.

Teeth and feeding aparants

The front portion of the jaws formed a toothless beak. Behind this beak were rows of relatively small, leaf-shaped teeth.

These teeth were considerably less impressive than those of large carnivorous dinosaurs such as Tyrannosaurus. They were adapted to processing vegetation rather than tearing flesh.

The broad muzzle suggests that Ankylosaurus was probably a relatively non-selective browser rather than an animal that carefully selected individual leaves or fruits.

Ankylosaurs as a group appear to have primarily consumed vegetation close to the ground. Their body structure suggests that they were generally adapted to feeding at relatively low heights rather than reaching high into trees.

Possible food sources included:

ferns;
low-growing shrubs;
cycads;
other low vegetation;
and potentially flowering plants available in Late Cretaceous ecosystems.

The large abdominal region of ankylosaurs is consistent with a digestive system capable of processing large quantities of relatively low-quality plant material through fermentation.

Armor

The most famous feature of Ankylosaurus was its armor.

Its body was covered with osteoderms, structures consisting of bone that developed within the skin rather than being directly connected to the skeleton.

These osteoderms could vary considerably in shape and size.

The armor included:

small ossicles;
larger plates;
protective structures around the neck;
and heavily developed armor along the body.

The neck was particularly well protected by rows of bony plates arranged into cervical half-rings.

This armor would have provided substantial protection against attacks from predators.

However, armor was probably not simply a passive shield. The arrangement and shape of osteoderms may also have influenced how the animal appeared to members of its own species, potentially playing a role in visual signaling or species recognition.

Tail Club

The tail club is perhaps the single most recognizable feature of Ankylosaurus.

The tail was reinforced by a series of specialized vertebrae and supporting structures. At its end was a large, rigid bony club.

The tail club was not simply a loose mass of bone. It was integrated into a mechanically reinforced tail capable of transferring considerable force.

Defensive function

The most widely accepted interpretation is that the tail club served as a defensive weapon.

A threatened Ankylosaurus could potentially swing its tail toward an attacking predator. A powerful strike could have caused serious injury to an attacking theropod.

This would have been particularly important in the Late Cretaceous ecosystems of western North America, where large predators such as Tyrannosaurus were present.

Intraspecific combat

Another possibility is that tail clubs could have been used during conflicts between members of the same species.

Individuals might have competed over:

territory;
food;
mates;
or social status.

However, the exact importance of tail-club combat remains uncertain.

The mechanical structure of the tail supports the idea that it was capable of delivering powerful blows, but determining precisely how frequently it was used against predators versus other Ankylosaurus requires additional evidence.

Locomotion

Ankylosaurus was a quadruped, meaning that it walked on all four limbs.

Its limbs were short but exceptionally robust.

The forelimbs appear to have carried a substantial proportion of the animal's body weight. Research on ankylosaur anatomy suggests that their forelimbs were particularly powerful, while their hind limbs were proportionally less dominant than in many other quadrupedal dinosaurs.

Its feet were broad and adapted to supporting its enormous mass.

Although often portrayed in older artwork as extremely slow and almost incapable of rapid movement, Ankylosaurus was probably capable of making relatively quick movements when necessary. It was unlikely to have been a fast-running dinosaur, but speed was not its primary defense.

Instead, its strategy appears to have been based on:

armor + low body profile + powerful tail weapon.

Defence

The defensive system of Ankylosaurus was remarkably sophisticated.

A large predator approaching the animal would have encountered several obstacles.

Low body profile

Its body was close to the ground, making it difficult for a predator to reach vulnerable areas.

Heavy armor

The osteoderms provided extensive protection over the body.

Armored neck

The cervical armor helped protect the relatively vulnerable connection between the skull and torso.

Reinforced skull

The heavily constructed skull could withstand substantial mechanical stress.

Tail club

The animal possessed an active weapon capable of striking an attacker.

Together these adaptations made an adult Ankylosaurus an extremely difficult prey item.

A predator might therefore have preferred younger, injured, or otherwise vulnerable individuals rather than attempting to attack a healthy adult.

Diet

Ankylosaurus was a herbivore.

Its diet probably included a variety of low-growing vegetation.

Possible food sources included:

Ferns
Leaves
Soft shoots
Low-growing plants
Other available vegetation

Its low body position would have allowed it to feed close to the ground.

It did not need to climb or reach high vegetation like some long-necked dinosaurs.

Feeding Behavior

The feeding behavior of Ankylosaurus is still debated.

Its teeth were relatively simple and its jaws were not capable of the highly efficient chewing seen in some herbivorous mammals.

It probably cropped vegetation using its jaws and then relied on its digestive system to break down plant material.

Its broad body may have provided enough space for a large digestive tract.

This would have helped it process relatively low-quality plant material.

Habitat

Ankylosaurus lived in western North America during the Late Cretaceous.

At this time, the region was very different from today.

The landscape included:

Rivers
Floodplains
Forested areas
Wetlands
Coastal environments

Western North America was part of a large landmass known as Laramidia.

Laramidia formed the western portion of North America during the Late Cretaceous, while a large inland seaway separated it from the eastern portion of the continent.

Predators

Ankylosaurus lived alongside some of the most famous predators of the Late Cretaceous.

These included large tyrannosaurids such as Tyrannosaurus rex.

A fully grown Ankylosaurus would have been a dangerous target because of its armor and tail club.

Young or injured individuals may have been more vulnerable.

A predator attacking an adult would have needed to find an unprotected area or attempt to overturn the animal.

Respiratory System

Like other dinosaurs, Ankylosaurus likely possessed a highly efficient respiratory system.

Evidence from other dinosaur groups suggests that many dinosaurs had air-sac systems that helped move air through the lungs.

However, the exact respiratory anatomy of Ankylosaurus is less well understood because soft tissues are rarely preserved.

Its breathing system probably shared some characteristics with other ornithischian dinosaurs.

Growth and Development

Like other dinosaurs, Ankylosaurus grew considerably during its lifetime.

Young individuals were probably much smaller and less heavily built than adults.

The armor likely developed progressively as the animal matured.

Juveniles may therefore have had a different body shape and defensive capabilities compared with fully grown individuals.

Because fossil specimens are limited, the exact growth pattern of Ankylosaurus remains uncertain.

Behavior

The behavior of Ankylosaurus cannot be observed directly, so paleontologists must infer it from anatomy and fossil evidence.

It was probably primarily a terrestrial animal that spent much of its time feeding.

There is no strong evidence that it lived in large herds.

It may have been solitary or lived in small groups, but this remains uncertain.

Its defensive anatomy suggests that avoiding predators may have been more important than actively confronting them.

Reproduction

Like other dinosaurs, Ankylosaurus reproduced by laying eggs.

However, direct evidence of Ankylosaurus nesting behavior is limited.

There are no well-known nesting colonies that can confidently be assigned to the species.

As a result, many details about its reproduction remain unknown.

Extinction

Ankylosaurus lived until the very end of the Cretaceous Period.

It disappeared approximately 66 million years ago, during the mass extinction event at the end of the Cretaceous.

The extinction event eliminated all non-avian dinosaurs.

The leading explanation is the impact of a large asteroid near what is now the Yucatán Peninsula in Mexico.

The impact caused major environmental changes, including changes in climate and food availability.

Other Ankylosaurs

Ankylosaurus was part of a diverse group of armored dinosaurs known as ankylosaurs.

Other well-known ankylosaurs include:

Euoplocephalus
Anodontosaurus
Edmontonia
Zuul
Borealopelta

These dinosaurs shared characteristics such as armor and low-slung bodies, but their armor, skulls, tails, and body proportions varied considerably.

Some ankylosaurs had large tail clubs, while others had different defensive structures.

Evolutionary Relationships

Paleoecology

The ecosystem inhabited by Ankylosaurus contained a wide range of organisms.

Plants formed the base of the food web, supporting numerous herbivorous dinosaurs.

These included:

Hadrosaurs
Ceratopsians
Ankylosaurs

Large predators such as tyrannosaurids occupied the upper levels of the food chain.

The environment was therefore highly diverse, with many herbivores competing for plant resources and predators interacting with them.

The exact evolutionary relationships among Late Cretaceous ankylosaurs have changed as new fossils and analyses have become available.

Ankylosaurus is generally placed within Ankylosauridae.

It belongs to a later branch of armored dinosaurs that developed increasingly specialized defensive structures.

Its combination of heavy armor, broad body, and tail club represents one of the most specialized defensive designs among dinosaurs.

Taphonomy

The relative rarity of complete Ankylosaurus skeletons is important when interpreting its anatomy.

Bones and armor plates can become separated after death.

Because osteoderms were embedded in the skin rather than connected directly to the skeleton, they could become scattered during decomposition and transport.

This makes it difficult to reconstruct the exact arrangement of armor from incomplete specimens.

Paleontological Importance